Showing papers by "S. D. Hunsberger published in 2008"

The First Swift Ultra-Violet/Optical Telescope GRB Afterglow Catalog

Abstract: We present the first Swift Ultra-Violet/Optical Telescope (UVOT) gamma-ray burst (GRB) afterglow catalog. The catalog contains data from over 64,000 independent UVOT image observations of 229 GRBs first detected by Swift, the High Energy Transient Explorer 2 (HETE2), the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and the Interplanetary Network (IPN). The catalog covers GRBs occurring during the period from 2005 Jan 17 to 2007 Jun 16 and includes ~86% of the bursts detected by the Swift Burst Alert Telescope (BAT). The catalog provides detailed burst positional, temporal, and photometric information extracted from each of the UVOT images. Positions for bursts detected at the 3-sigma-level are provided with a nominal accuracy, relative to the USNO-B1 catalog, of ~0.25 arcseconds. Photometry for each burst is given in three UV bands, three optical bands, and a 'white' or open filter. Upper limits for magnitudes are reported for sources detected below 3-sigma. General properties of the burst sample and light curves, including the filter-dependent temporal slopes, are also provided. The majority of the UVOT light curves, for bursts detected at the 3-sigma-level, can be fit by a single power-law, with a median temporal slope (alpha) of 0.96, beginning several hundred seconds after the burst trigger and ending at ~1x10^5 s. The median UVOT v-band (~5500 Angstroms) magnitude at 2000 s for a sample of "well" detected bursts is 18.02. The UVOT flux interpolated to 2000 s after the burst, shows relatively strong correlations with both the prompt Swift BAT fluence, and the Swift X-ray flux at 11 hours after the trigger.

Swift-UVOT captures the earliest ultraviolet spectrum of a Gamma Ray Burst

Abstract: We present the earliest ever ultraviolet spectrum of a gamma-ray burst (GRB) as observed with the Swift-UVOT. The GRB 081203A spectrum was observed for 50 seconds with the UV grism starting 251 seconds after the Swift-BAT trigger when the GRB was of u ~13.4 mag and still rising to its peak optical brightness. The UV grism spectrum shows a damped Ly-alpha line, Ly-beta, and the Lyman continuum break at a redshift z = 2.05 +/- 0.01. A model fit to the Lyman absorption implies log N(HI) = 22.0 +/- 0.2 cm-2, which is typical for GRB host galaxies with damped Ly-alpha absorbers. This observation of GRB 081203A demonstrates that for GRBs brighter than v ~14 mag and with 0.5 < z < 3.5 the UVOT will be able to provide redshifts, and probe for damped Ly-alpha absorbers within 4-6 minutes from the time of the Swift-BAT trigger.

Extreme Properties of GRB 061007: a highly energetic or a highly collimated burst?

Abstract: GRB 061007 had the brightest optical afterglow observed by UVOT so far and a highly unusual afterglow light curve. It had an usually smooth and steep panchromatic temporal decay, which, from the start of the Swift observations, decayed as a power with a slope of α = 1.65±0.02 in the X‐ray and UV/optical bands. If we interpret the data in terms of a spherical blastwave, the energy budget that is implied is enormous, challenging the possible models for the progenitors. The alternative explanation is that with GRB 061007 we are witnessing the most collimated outflow ever observed. If this is correct, if implies the occurrence of a jet break within 80 s of the prompt emission. This event has therefore remarkable consequences for the GRB communicate, since it may imply that jet breaks can occur much earlier in the afterglow light curves than hitherto expected.

GRB 070318: A Case of Prompt Emission from the External Shock?

Abstract: The prompt emission of gamma‐ray bursts (GRBs) is generally thought to be produced by internal shocks within the relativistic outflow, while the longer‐wavelength afterglow is produced by external shocks resulting from the interaction between the outflow and external medium. This picture has gained support from the clear separation between the prompt and afterglow phases seen in most Swift GRBs. However, no separation is seen in GRB 070318. The prompt X‐ray emission extrapolated from the observed Swift BAT flux matches well the early X‐ray afterglow emission in both temporal and spectral properties, and the combined 0.3–10 keV light curve can be fit with a single power‐law with an index typical of late‐time afterglows. The UV‐optical light curve displays a similar power‐law decay after the broad maximum. In addition, the gamma‐ray emission displays a single, FRED(fast rise, exponential decay)‐like peak and unusually smooth light curve, consistent with a single impulsive event interacting with the external medium.